A digital microwave communication system has a merit that can be inexpensively constructed in a short period of time compared with a wired line such as an optical fiber. For this reason, the digital microwave communication system is generally employed as a backhaul link in a mobile network in recent years. And it is required for the backhaul link to further increase its capacity due to a development of a mobile broadband communication represented by an LTE (Long Term Evolution) and a mobile WiMAX (Worldwide Interoperability for Microwave Access).
In the digital microwave communication system, a link budget analysis is generally carried out according to a standard that is specified to G.826 or the like standardized by ITU-T (International Telecommunication Union-Telecommunication Standardization Sector). Therefore, in most cases, the digital microwave communication system is operated in a state having a plenty of room for a communication line quality through a year. In recent years, in order to improve more utilization efficiencies and availabilities of the line, an adaptive modulation scheme is coming to be used, which changes over a modulation scheme suitably depending on a state of a transmission path (e.g., patent literature 1 is referred to).
As to the adaptive modulation scheme disclosed in the patent literature 1, an appropriate modulation scheme is selected at a reception side using a reception signal level or error rate information of a demodulated signal, and a selected result is transmitted to an opposite station side. At the opposite station side, a transmission modulation scheme is changed over depending on the selected result. For example, if a transmission path state is favorable, a modulation scheme with a higher transmission rate such as a multi-level modulation scheme is applied. Accordingly, the adaptive modulation scheme makes it possible to increase a communication capacity compared with a system in which a modulation scheme is fixed. On the contrary, if the transmission path state is quite unfavorable, a modulation scheme capable to respond to its environment such as a modulation scheme with a small modulation multi-level number is applied as the adaptive modulation scheme. Therefore, since it is possible to prevent a line disconnection, line operation rate can be increased.
In the radio communication system using the adaptive modulation scheme, it is very important to control a transmission power because an appropriate reception level needs to be kept in different modulation schemes. For example, a transmission power control method in the radio communication system using the adaptive modulation scheme is disclosed in patent literatures 2 and 3.
In the transmission power control method disclosed in the patent literature 2, an amount of the attenuation of the transmission line is obtained from a Carrier to Noise Ratio (hereinafter referred to as “CNR”) of a presumed reception signal at a reception side and a transmission power information multiplexed in the reception signal. And the reception side controls the transmission power of the signal transmitted to equipment of an opposite side according to the obtained attenuation so as to satisfy the required CNR in the modulation scheme at that time.
The transmission power control method disclosed in the patent literature 3 determines the modulation scheme and the transmission power of a signal transmitted to equipment on the opposite side based on a difference between a current reception level and a predetermined required reception level.
A method to control the modulation scheme and the transmission power of the reception signal based on the transmission path state is also disclosed in patent literature 4.
There is also an adaptive modulation scheme which controls the modulation scheme and the transmission power at the opposite station (e.g., patent literature 5 is referred to). The adaptive modulation scheme disclosed in the patent literature 5 requires a modification of the modulation scheme and the transmission power for an opposite side equipment based on a difference between the current reception level and a predetermined required reception level.